I had a question via email asking about Comet Holmes. I thought that by answering it on the blog, maybe others would also have some questions answered. so, thanks to Marycie for her question.
Comet Holmes was a very dim, and expected comet. Until October last year. During the period October 23rd to 24th 2007, it suddenly brightened. If you want to know more about why it suddenly became visible, I’d recommend reading Astroprof’s post on Integrated Magnitudes.
The comet was visible to the naked eye after that date, and on about the 26th of October it began to look more like a classic comet, with a tail and nucleus. Comets have a nucleus, a tail and a coma, or halo. The nucleus is the hard, chewy centre. This is rocky bit and really ‘is’ the comet, if you like. The tail and coma are produced when the object passes closer to the Sun and particles of ice and dust begin to sublimate. A cloud of material (the coma) appears to boil off the nucleus and becomes the glowing, cloudy ball that makes comets familiar to most people.
The tail is produced by interaction with the Sun. Particles from the coma and nucleus are blasted back from the comet by the Sun and so the tail always points away from the Sun.
Comet Holmes’ coma grew enormously in size and this became interesting later on in October. Late in the month, the coma had grown to be about half the width of the Moon on the sky. However it was lying about twice as far from the Earth as the Sun is (2 AU), so the true size was around 1 million km. That’s two-thirds the diameter of the Sun.
Here’s an animation from the Comet Holmes Wikipedia entry, showing the location of the comet and the size of the coma on different dates.
In November 2007 the coma became even larger and was in fact bigger than the Sun. However because of this, the coma has become so diffuse that it was hardly visible to the naked eye at all. Although much of the internet was talking about Comet Holmes being bigger than the Sun, most general news and media didn’t care because they couldn’t see it well enough to show the public a good picture.
The final part of Marycie’s question was asking where in the solar system the comet was located. Well I mentioned earlier that it was 2AU away from us. For the time it was visible, Comet Holmes sat just beyond the orbit of Mars. You see its location in this video I’ve made using the excellent Starry Night software.
This video covers the period October 2007 to April 2008. You’ll note that the Earth moves half an orbit during the video. Apologies for the low quality of YouTube.
[youtube:hBp://www.youtube.com/watch?v=U7pwBVuTgwE 351 291]
There is an excellent composite photo of Comet Holmes over the course of the 2007-2008 outburst on the Wikipedia page covering the topic. This image is shown below.